1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus for measuring the concentration of gases which may be employed in an air-fuel ratio control system for automotive vehicles, and more particularly to a gas concentration measuring apparatus designed to measure two kinds of gas components and offset an error in measuring one of the gas components which depends upon the other gas component.
2. Background Art
The air pollution caused by exhaust emissions of automotive internal combustion engines is giving rise to a serious problem at the present day. The exhaust emission control standard regulations have been made more rigorous recently. The burning control of gasoline or diesel engines or use of catalyst are, therefore, being studied to reduce pollutants contained in exhaust gasses. In U.S., OBD-II (On Board Diagnostic-II) requirements prescribe that automotive vehicles have a function of determining whether a catalytic converter is operating normally or not.
As one of systems meeting the OBD-II requirements, a two-O.sub.2 sensor monitoring system is proposed which monitors outputs of two O.sub.2 sensors mounted upstream and downstream of a catalytic converter, respectively, but it is not designed to detect pollutants directly and cannot determine whether pollutants in exhaust gasses have been reduced or not accurately.
If it becomes possible to measure the concentration of NOx in exhaust gasses for monitoring the burning control and the catalytic converter, the pollutants in the exhaust gasses can be reduced greatly. Specifically, the reduction in pollutants in exhaust emissions of the engine is achieved by controlling the quantity of fuel to be injected into the engine and the EGR rate based on the concentration of NOx contained in the exhaust gasses. Additionally, the determination of deterioration of the catalytic converter is achieved easily by installing a NOx sensor downstream of the catalytic converter.
NOx sensors capable of measuring the concentration of NOx accurately and techniques for mounting such NOx sensors in automotive vehicles are, therefore, being sought.
The effects of air-fuel ratio feedback control may be improved further by monitoring the concentration of O.sub.2 contained in the exhaust gasses as well as the concentration of NOx. Specifically, modern air-fuel ratio control for automotive vehicles is required to improve the accuracy of the control and perform lean burn engine control. For meeting these requirements, sensors capable of determining the air-fuel ratio of a mixture supplied to the engine over a wide range are being sought.
U.S. Pat. No. 5,866,799 teaches a NOx sensor designed to reduce the quantity of O.sub.2 contained in exhaust gasses first and then measure the concentration of NOx in the exhaust gasses. The measurement of concentration of NOx is achieved by decomposing NOx gas components into oxygen ions and measuring an electric current produced by the flow of the oxygen ions through electrodes. This type of NOx sensor, however, has the drawback in that the part of O.sub.2 gas contained in the exhaust gasses entering the sensor reaches the electrodes for measuring the concentration NOx, which causes an error component to be produced in the current indicative of the concentration of NOx. This problem will also be referred to in detail later in description of embodiments of the invention.